Quantum Chemistry Study On The Surface Properties And Dispersion Of Kaolinite And Montmorillonite

Kaolinite and montmorillonite are the main inorganic minerals in coal slime flotation.They are easy to form fine particles in water.In the flotation process,the fine clay particles can coagulate with the purpose minerals(coal particles)and then cover on their surfaces.Also,the fine clay particles can be carried into concentrate with foam water and flotation bubble.As a result,the selectivity of mineral separation is affected and the quality of flotation concentrate is reduced.Therefore,to study the surface properties and dispersion of kaolinite and montmorillonite is helpful to find and explore the efficient dispersion inhibitors.These work play an important role in improving of flotation selectivity and concentrate grade.In this paper,quantum chemical methods are applied to study crystal structures,surface properties and the interactions between minerals and inhibitors from the microcosmic viewpoint.Meanwhile,the actual effect of the agents were verified by the dispersion tests and the potential tests.The main content is listed below: Firstly,characterize the properties of the crystal structure and surface structure of kaolinite and montmorillonite;Secondly,study the adsorption behaviors of Ca2+ ion,water molecule,sodium silicate species and hexametaphosphate on main cleavage(001)surfaces of kaolinite and montmorillonite in alkaline pH;Thirdly,study the frontier orbital properties of nine kinds organic carboxylates such as citrate and acetate and the adsorption behaviors of carboxylates(taking acetic acid as an example)on minerals(001)surfaces;Lastly,study the dispersion effects of sodium silicate,sodium phosphate and sodium carboxylates on kaolinite and montmorillonite in the presence or absence of Ca2+.Based on the above research,the main conclusions are as follows:(1)Kaolinite and montmorillonite crystals are easily cleavage along(001)plane.Hydrogen bonding is the main force between kaolinite interlayers,formed by hydrogen atoms of alumina octahedron and oxygen atoms of silicon tetrahedron.While Van der Waals forces of two adjacent layers and electrostatic forces of counterions with negatively charged silicon tetrahedron are the main forces between montmorillonite interlayers.There are imbalances of local charge in the internal structures of kaolinite and montmorillonite.For kaolinite crystal,the silicon tetrahedron contains more negative charge and the alumina octahedron contains positive charge.The charge properties of the montmorillonite crystals are mainly reflected in the interlayer counterions and the network structure.The orbital reactivity of kaolinite and montmorillonite are controlled by oxygen atoms,and the Na atom in the montmorillonite also has an important influence on the frontier orbitals.(2)The charge properties of surface atoms change after the formation of(001)cleavage surfaces.Kaolinite Al-(001)surface shows more positive charge and the hydroxyl which parallel to the(001)plane is an active site.The Al-(001)surface prone to adsorb anion in solution.The oxygen atoms on the bottom surface of Si-(001)surface have strong reaction activity.Cations can easily be adsorbed on this surface.The properties of montmorillonite None-(001)surface are similar to kaolinite Si-(001)surface.In order to achieve charge balance,None-(001)surface needs to adsorb some cations.(3)The solution environment has an important influence on the dispersion of kaolinite and montmorillonite.Their surface are strong hydrophilic.The hydrophilicity of kaolinite is mainly reflected in the Al-(001)surface.Hydrogen and oxygen atoms of surface and water molecule form strong hydrogen bonds.The montmorillonite Na-(001)surface is hydrophilic compared with None-(001)surface.This is due to the strong hydration of Na ions of Na-(001)surface.The calcium ions in solution can be adsorbed on the surface of two kinds of minerals.Hydroxy calcium and mineral surfaces bearing negative electricity have strong electrostatic effects.The DFT calculated results show that the absolute value of adsorption energies of hydroxyl calcium are greater than that of water molecule.This means that hydroxyl calcium is able to repel water molecules on the surface and adsorb on the surface.The experimental results show that the absolute value of surface potential of mineral slurry decreases with the presence of calcium ions.And the dispersion of minerals slurry is extreme poor.(4)Anionic inhibitors such as sodium silicate,sodium hexametaphosphate and sodium acetate can adsorb on the surface of kaolinite and montmorillonite.Their adsorption behavior mainly occurred on the kaolinite Al-(001)surface and montmorillonite Na-(001)surface.The oxygen atoms of adsorbates form covalent hydrogen bonds with the hydrogen atoms of kaolinite Al-(001)surface.Meanwhile,the surface transfers some electronics to anions,and the adsorbate form a certain electrostatic force with the surface.For the montmorillonite Na-(001)surface,oxygen atoms of absorbates form strong electrostatic forces with the Na atoms of the surface.The adsorption energies of three kinds of anions on kaolinite surfaces are lower than that of montmorillonite.That is to say,the dispersion effect of regents on kaolinite is better.Among these three regents,the effect of sodium hexametaphosphate is the best.(5)The structure of the organic carboxylate inhibitors were studied,and it was found that their HOMO orbitals were located in the carboxyl groups.The reagents containing more polar groups have higher reactivity and can be adsorbed on the mineral surface stably.Calculation results show that the citric acid and the hydrogen phthalate among the 9 kinds of organic carboxylic acids have higher reactivity with kaolinite and montmorillonite.(6)The dispersion effect of sodium silicate,sodium hexametaphosphate and 4 kinds of organic carboxylate on kaolinite and montmorillonite were tested.The results show that sodium hexametaphosphate and sodium citrate have good dispersion effect on two minerals.Because of montmorillonite's strong hydrophilicity,the dispersion effect of the two inhibitors is more obvious in the presence of calcium ions.These conclusions are consistent with the DFT simulation results.In this paper,the surface properties of kaolinite and montmorillonite were quantified by quantum chemical methods.Interactions between commonly used inhibitors in flotation practice and minerals surfaces were simulated by DFT method.The laws of mechanism of inhibitors on kaolinite and montmorillonite were recognized from the microscopic view point.And two kinds of reagents with good dispersing effect on clay minerals were screened out.The microscopic simulation results have a good match with the macroscopic experimental phenomena.The results from this study indicate that quantum chemistry simulations may provide a route to the identification or even design of particular organic surfactants for use in mineral separation by flotation.